1. Field of the Invention
This invention relates to a semi-conducting ceramic which is useful in a jet engine igniter for a high energy, low voltage ignition system which includes a capacitor. A voltage applied to such an ignition system, when in operation, charges the capacitor and applies an increasing voltage to a spark gap of the igniter between a center electrode and a ground electrode. The ceramic of the instant invention is incorporated in the igniter so that a portion of a surface thereof is adjacent the spark gap. When the voltage applied to the igniter reaches a sufficiently high value, a spark discharge occurs between the center and ground electrodes of the igniter. This discharge ionizes the spark gap, thereby enabling a continuing current to flow until the capacitor in the ignition system is discharged. It is the discharge of the capacitor that makes this a "high energy" ignition system.
Igniters for use in high energy ignition systems are also known in which a portion of a surface of an insulating ceramic is adjacent the spark gap between the center and ground electrodes. Other factors being equal, when the ceramics are clean, the voltage required to initiate sparking is less when a semi-conducting ceramic is adjacent the spark gap than when an insulator is adjacent. It is the semi-conducting ceramic of the instant invention that makes an igniter produced therefrom, as described above, one of the "low voltage" type.
2. The Prior Art
Various electrically conducting ceramic bodies have heretofore been suggested and used in igniters for low voltage ignition systems; see, for example, U.S. Pat. Nos. 3,037,140 and 3,046,434 for early examples of ceramics of this type. Igniters made with these ceramics were used in engines where the operating conditions were not particularly severe. Improved semi-conductor bodies made by hot pressing mixtures of alumina and silicon carbide are disclosed in U.S. Pat. No. 3,558,959. The hot pressed alumina silicon carbide semi-conducting ceramics performed satisfactorily under more severe operating conditions, but their production proved to be complicated and expensive. For example, it was not found to be possible to produce bodies of the required shape by the hot pressing technique; instead, the extremely hard alumina silicon carbide body was shaped, after hot pressing, by boring, honing and grinding with diamond tools.
U.S. Pat. Nos. 3,968,057 and 4,120,829 disclose that a satisfactory semi-conducting ceramic can be produced without hot pressing or extensive shaping operations by pressing a batch of silicon carbide, alumina, and various calcium and magnesium compounds into a shape, and firing the shape. The method of the former patent involves two firings, a first in air, and a second in an inert atmosphere to produce the semi-conductor body. The method of the latter patent involves only a single firing, in an inert gas atmosphere.
It has also been suggested (see, for example, U.S. Pat. Nos. 3,376,367 and 3,573,231) that semi-conducting ceramics can be produced by embedding a body of a desired shape, and comprising silicon carbide and aluminum silicate or the like, in a mass of silicon carbide particles, and firing. The aluminum silicate can be a part of the batch from which the original shape is formed, charged as such, or it can be produced in situ in a shape pressed from a mixture of silicon carbide and alumina by firing the shape to oxidize a part of the silicon carbide to silicon dioxide which can then react with alumina in the shape to produce the aluminum silicate.
Finally, U.S. Pat. No. 3,052,814 suggests the production of semi-conducting ceramics by nitriding the silicon in shapes pressed from mixtures of silicon carbide and silicon. The patent also indicates that the semi-conducting ceramics can be used in the production of igniters.